This invention relates to the manufacture of a monolith-type automotive catalytic converter comprising a wound foil structure formed of a heat resistant Al-containing ferritic alloy foil carrying a catalyst-bearing alumina coating. More particularly, this invention relates to preparing an Fe-Cr-Al-Y foil to bond with improved adhesion an alumina coating, which coated foil is suitable for forming a monolith-type catalytic converter.
Monolith catalytic converters are known for treating automotive exhaust gases to reduce noxious emissions. The monolith converter typically comprises an extruded ceramic cylindrical structure having a plurality of square axial passageways in a cross-sectional honeycomb pattern. The passageway surfaces bear the noble metal catalyst, such as a Pt/Pd/Rh mixture, which catalyst interacts with exhaust gas caused to flow therethrough. A similar converter structure is formed of cold rolled metal foil having a catalyst-impregnated ceramic coating. The foil is corrugated and wound into a cylindrical structure wherein the corrugations form gas-conducting passageways. When compared to the monolith, the wound foil structure features substantially thinner walls, which permits an increase in the catalyst-bearing surface area per unit volume, thereby improving the converter efficiency.
The foil for the converter is suitably composed of a high temperature corrosion-resistant, Al-containing ferritic alloy. A preferred alloy consists of, by weight, 15 to 25% chromium (Cr), 3 to 6% aluminum (Al), 0.3 to 1.0% yttrium (Y) and the balance iron (Fe). The alloy, which is conveniently designated Fe-Cr-Al-Y, is readily commercially available as cold-rolled foil. The foil is typically pretreated by heating in air at 1000.degree. C. for between 1 to 24 hours to form a thin, tightly adherent oxide surface layer which protects the underlying alloy from further oxidation during high temperature use. The protective layer is composed predominately of alpha alumina, but also contains a small amount of yttrium oxide, which improves the adhesion of the oxide layer to the alloy substrate. When viewed with a scanning electron microscope, the surface of the protective layer is substantially flat or may comprise slight nodules.
The protective oxide layer is not satisfactory for carrying the catalyst. Therefore, the oxidized foil is coated with an aqueous alumina dispersion and fired to form a high surface area gamma-alumina coating, which is impregnated with the catalyst. The applied alumina coating provides a large number of suitable catalyst metal sites and also reduces the detrimental effects of some exhaust gas constituents upon the catalyst performance.
When the wound foil converter is incorporated into an automobile exhaust system for treating exhaust gas, the catalyst-bearing coating tends to spall. The spalling problem is fundamentally related to poor adhesion of the applied alumina coating to the protective oxide layer. Adhesion is poor partly because the smoothness of the protective layer surface does not enable it to satisfactorily grip or anchor the applied coating. Thus, the coating breaks off when the converter is subjected to mechanical vibration and cyclic temperature operation, conditions typically encountered during its automotive use. Cycling between ambient and exhaust gas temperatures is particularly detrimental because of the difference in thermal expansion between the alloy and the alumina coating. The overall result is that the coating spalls and converter performance is impaired.
Therefore, it is an object of this invention to provide an improved monolith-type automotive catalytic converter formed of a metal foil of an Al-containing ferritic stainless steel alloy, which foil is covered by a protective oxide surface layer and an overlying catalyst-bearing alumina coating. The coating exhibits improved resistance to spalling. The protective oxide layer is formed of densely spaced protruding crystalline whiskers that substantially cover the alloy surface and tightly bond the applied coating.
It is a further object of this invention to provide a foil composed of an Fe-Cr-Al-Y alloy or a similar Y-free alloy and formed by a metal peeling process, which foil surface is substantially covered by a corrosion-protective oxide made up of densely spaced protruding crystalline whiskers. The whisker-covered surface is adapted to bond with improved adhesion a ceramic or other coating applied thereto. In one particularly useful aspect of this invention, a catalyst-impregnated alumina coating is applied to the whisker-covered foil to produce a spall-resistant coated foil suitable for manufacturing a monolith-type catalytic converter.
It is also an object of this invention to provide a method of producing a metal foil formed of an Fe-base alloy containing Al and Cr and capable of being coated with improved adhesion, which method comprises oxidizing severely cold worked foil by heating in an oxygen-containing atmosphere to form on the surface thereof an oxide layer substantially made up of protruding whiskers.
It is a more particular object of this invention to provide a method of preparing a metal foil to receive a coating with improved adhesion, which method comprises forming by a metal peeling process a foil of an Fe-base alloy containing Cr, Al and preferably Y and treating the peeled foil by heating at a specific temperature range in the presence of oxygen to grow a tightly-adherent protective oxide layer comprising high aspect oxide whiskers. The whiskers substantially cover the foil surface and improve the bonding of a subsequently applied coating. In one aspect of this invention, the whisker-covered Fe-Cr-Al-Y foil is coated with an alumina-base coating and is used to manufacture a monolith-type catalytic converter having improved spall resistance.